1. Field of the Invention
The invention pertains to molecular imprinting of small particles and, more particularly, to a method of molecular imprinting which utilizes a propellant as the solvent and dispersing agent of the matrix material and to imprinted particles formed by the method as well as devices coated with imprinted particles, such as, for example, surface acoustic wave (SAW) devices. In addition, the invention pertains to a method for the formation of small particles of monomers containing solid-state reactivity.
2. Description of the Prior Art
Molecular imprinting is a process, which involves arranging of polymerizable functional monomers around a template (print) molecule. This is achieved either by utilizing non-covalent interactions such as hydrogen bonds, ion-pair interactions, etc. (non-covalent imprinting), or by reversible covalent interactions (covalent imprinting) between the print molecule and the functional monomers. Typically, a molecule to be imprinted (template) is combined with a mixture of functionalized and non-functionalized monomers so that the monomers surround the template. In the process, functionalized monomers align themselves in a binding relationship to complementary functional groups on the template to form therefore a complex with the template. After polymerization, functional groups are held in position by the highly cross-linked polymeric matrix. The template is then removed, and the resulting material contains imprinted binding sites which are complimentary in size and shape to the template. The complementary binding groups, arising from the functionalized polymer groups incorporated during the imprinting, are specifically positioned to enhance the preferential substrate binding and, if desired, subsequent catalysis. The imprinted polymer materials are capable of specific sorption or specific catalytic activity. A good description of state of the art of molecular imprinting can be found in Mosbach, K., Trends in Biochemical Sciences, Vol. 7, pp. 92-96, 1994; Wulff, G., Trends in Biotechnology, Vol. 11, pp. 85-87, 1993; and Andersson, et al., Molecular Interactions in Bioseparations (Ngo. T. T. ed.), pp. 383-394.
The functionalized monomers usually used for molecular imprinting are: acrylic acids [Anderson, L.; Sellergren, B; Mosbach, K Tetrahedron Lett. 1984, 25, p. 5211. Sellergren, B.; Lepisto, M.; Mosbach, K. J. Am. Chem. Soc. 1988, 110, p. 5853. Andersson, L. I.; Mosbach, K. J. Chromatogr. 1990, 516, p. 313. Matsui, J.; Miyoshi, Y.; Takeuchi, T. Chem. Lett. 1995, p. 1007.], vinylbenzoic acids [Andersson, L.; Sellergren, B.; Mosbach, K. Tetrahedron Lett. 1984, 25, p. 5211], acrylamino-sulfonic acids [Dunkin, 1. R.; Lenfeld, J.; Sherrington, D. C. Polymer 1993, 34, p. 77], amino-metacrylamides [Beach, J. V.; Shea, K. J. J. Am. Chem. Soc. 1994, 116, p. 379.], vinylpyridines [Ramstrom, O.; Andersson, L. I.; Mosbach, K. J. Org. Chem. 1993, 58, p. 7562. Kempe, M.; Fischer, L.; Mosbach, K. J. Mol. Recognit. 1993, 6, p. 25], vinyl imidozales [Kempe, M.; Fischer, L.; Mosbach, K. J. Mol. Recognit. 1993, Vol. 6, p. 25. Leonhardt, A.; Mosbach, K. React. Polym. 1987, 6, p. 285.], acrylamides [Yu, C.; Mosbach, K. J. Org. Chem. 1997, 62, p. 4057.], and vinyl-iminodiacetic acids [Dhal, P. K.; Arnold, F. 14. J. Am. Chem. Soc. 1991, 113, p. 7417. Kempe, M.; Glad, M.; Mosbach, K. J. Mol. Recognit. 1995, 8, p. 35.].
Prior to this invention, methods of molecular imprinting have achieved only modest success in the enhancing polymer selectivity and catalytic activity. The reason for this is, that in order to be effective in a wide scale, imprinted materials must have binding/active sites to be homogeneous (in specificity and activity), be well formed (based on shape and reactivity), and be easily accessible by the reactant molecules (access is affected by shape, size and polarity of the channels leading to the catalytic site). The imprinted polymeric materials created by prior art methodologies have sites that are generally not very accessible and not homogenous, as they often have different binding affinities and/or reactivities. These problems mainly arise from the method used for producing the imprinted polymer particles.
A common method of molecular imprinting is referred to as solution polymerization. This method results in the formation of imprinted sites that are completely encased within the polymer. In order to enable an access to those sites, the polymer monolith must be subjected to mechanically grinding to produce particles that have exposed sites. Grinding produces irregularly shaped particles and typically only less than 50 percent (50%) of the ground polymer is recovered as useable particles with size less than 25 μm. Irregular particles generally give less efficient devices mainly because of the deformation of a large number of the binding sites. As a result, damage to the sites adversely affects their selectivity and activity. An alternative method to increase accessibility to the imprinted sites is by the use of porogen compounds which are known to generate foam-like polymer structures when combined with polymer forming materials. Porogens, which are typically inert solvents, are mixed with the polymerizable monomers during the imprinting process and are washed away after polymerization is complete. This creates large pores that allow access to the created binding sites. However, while the porogens are removed, some of the structural integrity of the polymer can be lost at the same time, leading to the deformation of the sites and loss in specificity and activity.
Another alternative for molecular imprinting is by direct polymerization of particles in liquid media. Surfactants are used to create molecular microstructures, such as micelles or reverse micelles. Then, inorganic or organic monomers are polymerized around those molecular microstructures at the surfactant-solvent interface to form polymer beads, dispersed in the liquid media to prevent agglomeration. The size and shape of the formed beads highly depend on the chemistry of the mixture and reaction conditions, such as temperature and stirring. When the surfactant is removed, the remaining material has a size and shape complementary to the size and shape of the initial molecular microstructures. By controlling variables such as surfactant selection and concentration, a variety of different microstructure shapes such as micellar, cubic, tetragonal, lamellar, tubular and reverse micellar can be formed. Consequently, monodisperse particles of a variety of different sizes and porous materials with a variety of different shapes of pores and channels can be created. Methods of making porous material are described, for example, in the following patents each of which are incorporated herein by reference: U.S. Pat. No. 5,250,282 to Kresge et al; U.S. Pat. No. 5,304,363 to Beck et al; U.S. Pat. No. 5,321,102 to Loy et al; U.S. Pat. No. 5,538,710 to Guo et al; U.S. Pat. No. 5,622,684 to Pennavaia et al; U.S. Pat. No. 5,750,085 to Yamada.
Molecular imprinting by direct polymerization of particles in liquid media is more advantageous, but still has limitations due to the liquid media needed to disperse particles to prevent particles agglomeration. Therefore, after polymerization, particles need to be separated from the liquid media for further use, which is not an easy task, especially for small particles. While in many applications, imprinted polymers should be deposited on the special surfaces, such as in chemical and biological sensors, and in chromatography and filtration devices. Deposition of the imprinted polymer material and adherence on the surface remains a big problem.
U.S. Pat. No. 5,587,273 to Yan et al., which is herein incorporated by reference, describes a way of molecular imprinting of polymer film directly on the surface of sensor. The invention describes molecularly imprinted substrate and sensors employing the imprinted substrate for detecting the presence or absence of analytes. One embodiment of the invention comprises first forming a solution comprising a solvent and (a) a polymeric material capable of undergoing an addition reaction with a nitrene, (b) a crosslinking agent (c) a functionalizing monomer and (d) an imprinting molecule. A silicon wafer is then spin coated with the solution. The solvent is evaporated to form a film on the silicon wafer. The film is exposed to an energy source to crosslink the substrate, and the imprinting molecule is then extracted from the film. Described method is an advance in deposition of imprinted polymers to the sensing surfaces. But there is no solution disclosed in the literature for imprinting of polymer particles directly on the surfaces of devices. Prior researchers have focussed on the preparation of imprinted particles, but not on attachment of the particles to the surfaces of device, and it would be advantageous to have a methodology which allowed direct attachment of imprinted particles to substrate surfaces.
Aerosol and vapor technology has been used for many industrial and medicinal applications which utilize particles. An aerosol is a two-phase system consisting of a gaseous continuous phase and a discontinuous phase of individual particles. The individual particles in an aerosol can be solids or liquids (Swift, D. L. (1985), “Aerosol characterization and generation,” in Aerosols in Medicine Principles, Diagnosis and Therapy (Moren, F. et al. eds) 53-75). Supercritical fluids have been used in the production of aerosols for precipitation of fine solid particles. The phenomenon was first observed and documented as early as 1879 and was described the precipitation of solids from supercritical fluids (Hannay, J. B. and Hogarth, J., On the Solubility of Solids in Gases, Proc. Roy. Soc. London, 1879, A29, 324). The sudden reduction in pressure reduces the solvent power of the supercritical fluid, causing precipitation of the solute as fine particles. This phenomenon has been exploited in many processes for producing fine particles, using co-solvents (Sievers, et al. PCT Publication WO 9317665 published Sep. 16, 1993, Donsi, G. and Reverchon, E. (1991), “Micronization by Means of Supercritical Fluids: Possibility of Application to Pharmaceutical Field,” Pharm. Acta Helv. 66:170-173), anti-solvents (Debenedetti, P. G., et al. (1993), “Application of supercritical fluids for the production of sustained delivery devices,” J. Controlled Release 24:27-44, PCT Publication WO 90/03782 of The Upjohn Company for “Finely Divided Solid Crystalline Powders via Precipitation Into an Anti-Solvent”, Yeo, S-D, et al. (1993), “Formation of Microparticulate Protein Powders Using a Supercritical Fluid Antisolvent,” Biotechnology and Bioengineering 41:341-346), as well as pure supercritical solvents (Mohamed, R. S., et al. (1988), “Solids Formation After the Expansion of Supercritical Mixtures,” in Supercritical Fluid Science and Technology, Johnston, K. P. and Penninger, J. M. L., eds., Tom, J. W. and Debenedetti, P. B. (1991), “Particle Formation with Supercritical Fluids—a Review,” J. Aerosol. Sci. 22:555-584, Smith U.S. Pat. No. 4,582,731 for “Supercritical Fluid Molecular Spray Film Deposition and Powder Formation,” issued Apr. 15, 1986, and Smith U.S. Pat. No. 4,734,451 for “Supercritical Fluid Molecular Spray Thin Films and Fine Powders). In the processes described, fine aerosols comprising the desired substance are formed by mixing a nongaseous pressurized or/and supercritical fluid(s) with the desired substance, which is present in a solution, dispersion, suspension, micellar system or emulsion. During rapid reduction of the pressure on composition the pressurized/supercritical fluids form a gas and a gas-borne dispersion of fine particles, liquid or solid.
There are many acronyms associated with those processes, including RESS, GAS or SAS, SEDS, ASES, and PGSS (Jennifer Jung, Michel Perrut Particle design using supercritical fluids: Literature and patent survey Journal of Supercritical Fluids 20 (2001) 179-219). RESS refers to Rapid Expansion of Supercritical Solutions. This process contemplates dissolving the product in the fluid and rapidly depressurizing this solution through a nozzle, causing an extremely rapid nucleation of the product into a highly dispersed material. GAS or SAS is Gas (or Supercritical fluid) Anti-Solvent, one specific implementation being SEDS (Solution Enhanced Dispersion by Supercritical Fluids). The general concept contemplates decreasing the solvent power of a polar liquid solvent in which the substrate is dissolved, by saturating it with carbon dioxide in supercritical conditions, causing substrate precipitation or re-crystallization. ASES is used when micro- or nano-particles are expected. The process contemplates pulverizing a solution of the substrate(s) in an organic solvent into a vessel swept by a supercritical fluid. SEDS is a specific implementation of ASES wherein there is co-pulverizing of the substrate(s) solution and a stream of supercritical carbon dioxide through nozzles. PGSS stands for Particles from Gas-Saturated Solutions (or Suspensions). The process includes dissolving a supercritical fluid into a liquid substrate, or a solution of the substrate(s) in a solvent, or a suspension of the substrate(s) in a solvent followed by a rapid depressurization of this mixture through a nozzle causing the formation of solid particles or liquid droplets.
Development of microspheres/capsules, containing a load of needed ingredient, is one of the most rapidly developing area in medicine, food industry, agrochemicals, cosmetics. Many efficient drugs have been reformulated to allow control of delivery location and rate, the active substance being distributed directly to the target to enhance the treatment efficiency and reduce the doses and related side effects. Some of the researchers classify particles/capsules smaller than 1 μm as nanoparticles and those larger than 1000 μm as macro-particles. Commercial particles/capsules typically have a diameter between 3 and 800 μm and contain 10-90 wt. % of carrier material. A wide range of materials have been embedded/encapsulated in microspheres/capsules, including adhesives, agrochemicals, live cells, active enzymes (W. Fischer, B. Muller, Patent EP 0 322 687, 17 Dec., 1988; P. Debenedetti, J. W. Tom, S. D. Yeo, G. B. Lim, Application of Supercritical Fluids for the Production of Sustained Delivery Devices. Journal of Controlled Release, 24, 1993, 27-44; L. Frederiksen, K. Anton, B. J. Barrat, P. Van Hoogevest, H. Leuenberger. Proceedings of the 3rd International Symposium on Supercritical Fluids; Tome 3; G. Brunner, M. Perrut (Eds.), ISBN 2-905-267-23-8, 17-19 October, Strasbourg, 1994, 235-240; M. Hanna, P. York, Patent WO 95/01221, 1994; M. Hanna, P. York, Patent WO 96/00610, 1995; K. Mishima, S. Yamaguchi, H. Umemoto, Patent JP 8-104830, 1996; P. Pallado, L. Benedetti, L. Callegaro, Patent WO 96/29998, 1996; W. Majewski, M. Perrut, Patent FR 99.12005, 27 September). Despite these advances, there are few materials which include an active agent embedded or encapsulated in a carrier matrix (or otherwise associated with the matrix) which are specifically designed for targeted delivery of the active agent to a particular site. It would be advantageous for example, if a material were available where a drug or toxin were associated with a slow release matrix material, wherein the material could be targeted for delivery to a tissue, organ or other site of activity, and then have slow sustained release at the targeted site. Prior to this invention, no such delivery material having each of these attributes existed.
There are several method of handling materials with solid state reactivity to develop small particles of reacted solid materials. There is a need to produce small particles which retain reactivity in the solid state. Reprecipitation in liquid solvents is one of the techniques used (Application: JP 92-238160 19920907 to Kasai; Oikawa H; Oshikiri T; Kasai H; Okada S; Tripathy SK; Nakanisbi H. Various types of polydiacetylene microcrystals fabricated by reprecipitation technique and some applications. POLYMERS FOR ADVANCED TECHNOLOGIES 2000, Vol 11, Iss 8-12, pp 783-790). The process is carried out by dissolving an organic material in a solvent, adding poor solvent, followed by crystallization or polymerization of the microcrystals to form particles. Adding 4-BCMU in EtOH solvents to water dropwise and irradiating with high-pressure Hg lamp gave polydiacetylene particles showing avarage diameter 100-200 nm. The reprecipitation method is a useful technique to fabricate organic microcrystals such as polydiacetylene (PDA), low-molecular-weight aromatic compounds, organic functional dyes that have features located in a mesoscopic phase between a single molecule and bulk crystals, and organic microcrystals which are expected to exhibit peculiar optical and electronic properties.
One known variation involves recrystallization in supercritical fluid by change of temperature and addition of antisolvents (Kasai, Hitoshi; Okazaki, Susumu; Okada, Shuji; Oikawa, Hidetoshi; Adschiri, Tadafumi; Arai, Kunio; Nakanishi, Hachiro. Fabrication of organic microcrystals by supercritical fluid crystallization method and their optical properties. MCLC S&T, Sect. B: Nonlinear Opt. (2000), 24(1-2), 83-88; Komai, Y; Kasai, H; Hirakoso, H; Hakuta, Y; Okada, S; Oikawa, H; Adschiri, T; Inomata, H; Arai, K; Nakanishi, H. Section 3: Thin Films—Size and Form Control of Titanylphthalocyanine Microcrystals by Supercritical Fluid Crystallization Method. Molecular Crystals and Liquid Crystals, 1998, v.322, p. 167, 6 p). This method involves the use of solvents which makes the particles thus produced only accessible in or subject to the solvent as an impurity. It would be advantageous to have particles and particle producing methods where both agglomeration and solvent impurities are completely avoided.